1. Field of the Invention
The present invention relates to an image forming method using the electrophotographic method, the electrostatic recording method, or the magnetic recording method. More particularly, the present invention relates to an image forming method applicable to an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, for forming an image by copying a toner image onto a copy medium after forming the toner image on a photosensitive member.
2. Description of the Related Art
Many conventionally known electrophotographic methods are generally based on a process comprising the steps of forming an electrical latent image on a photosensitive member by various means generally by the use of a photosensitive material, developing the electrical latent image with a toner into a visual image, transferring as required the toner image onto a transfer medium such as a sheet of paper, and then applying fixing heat or pressure, thereby obtaining a fixed image. In this process, the toner remaining on the photosensitive member, which is not transferred to the transfer medium after transferring, is cleaned off by various methods.
Known methods for visualizing the electrical latent image include the cascade developing method, the magnetic brush developing method, the non-magnetic one component developing method, and the pressurizing method. Further, a method known as the magnetic one component developing method is in use, which uses a magnetic toner, and causes an image to fly on an electric field between a photosensitive member and a sleeve by the use of a rotary sleeve having a magnetic pole arranged at the center.
LED and LBP printers primarily constitute the current printer market, and the technical tendency is toward a higher resolution, i.e., from the conventional 240 dpi or 300 dpi toward 400 dpi, 600 dpi or 1,200 dpi. A higher accuracy is accordingly demanded also for the developing method. General attention is attracted by the efforts to achieve a higher performance also in the area of copying machines, and the trend is directed toward more digitalized machines. Because a method of forming an electrostatic image by a laser is mainly adopted in the digital method, resolution is becoming higher, and accordingly, a developing method of a higher resolution and a higher accuracy in keeping with demands as in the area of printers is being realized.
Efforts are therefore directed toward achievement of a toner of a smaller particle size. A toner of a small particle size of a specific particle size distribution is proposed in Japanese Patent Application Laid-Open No. 1-112,253, No. 1-191,156, No. 2-284,6156, No. 2-284,158, No. 3-181,952, and No. 4-162,048.
There is at present proposed a method known as the contact one component developing method of conducting development with a configuration in which a semiconductive developing roller or a developing roller having a dielectric layer form on the surface thereof is pressed against a photosensitive surface layer.
When the photosensitive member is distant from the toner carrying member in the one component developing method, the electric force lines concentrate onto edge portions of the electrostatic latent image on the photosensitive member, and the toner is developed along the electric force lines, thus leading to an increased decrease in grade of the image caused by an edge effect in which the image is developed with the toner partial to the edge portions of the image.
This edge effect is prevented by bringing the photosensitive member and the toner carrying member closer to each other. It is, however, difficult to mechanically set a gap between the photosensitive member and the toner carrying member, i.e., to achieve a gap smaller than the thickness of the toner layer on the toner carrying member.
The edge effect would therefore be prevented by using the contact one component developing method by pressing the toner carrying member against the photosensitive member. However, when the displacement speed of the toner carrying member surface is the same as the displacement speed of the photosensitive member surface, visualization of the latent image on the photosensitive member cannot provide a satisfactory image. In the contact one component developing method, therefore, a part of the toner on the toner carrying member is developed for the latent image on the surface of the photosensitive member by providing a difference in surface displacement speed between the photosensitive member and the toner carrying member, and another part of the toner is stripped off, thus resulting in a developed image, free from the edge effect, resulting in a developed image that is highly true to the latent image.
In the application of the contact one component developing method, it is essential to provide a configuration permitting rubbing of the photosensitive member surface by the toner and the toner carrying member. As a result, the use of such a method for a long period of time tends to cause deterioration of toner, deterioration of the toner carrying member surface, and deterioration or wear of the photosensitive member surface, resulting in degradation of durability. There has therefore been a demand for an improved method. For there reasons, the contact one component developing method has essential problems including the necessity of improving durability, and difficulty of achieving a higher speed because of a larger load on the equipment.
Studies on the contact one component non-magnetic developing method are reported in the Japan Hardcopy '89 Papers, pages 25-28, without reference to durability.
An overview of a printer using the one component contact developing method is reported in the FUJITSU Sci. Tech., 28, 4, pp. 473-480 (December 1992). There remains, however, room for improvement regarding durability.
Japanese Patent Application Laid-Open Nos. 5-188,765 and 5-188,752 disclose techniques regarding a one component contact developing method, but does not disclose a technique for improving durability.
A toner image formed on the photosensitive member during the developing step is transferred, on the other hand, onto a transfer medium during the transferring step, while the toner remaining after transferring on the photosensitive member is cleaned during the cleaning step, and the toner is stored in a waste toner container. For the cleaning step, it has been the conventional practice to use a cleaning method such as blade cleaning, fur brush cleaning, or roller cleaning. All these cleaning methods comprise the steps of bringing a cleaning member into contact with the surface of the photosensitive member and mechanically scraping off the toner remaining from the photosensitive member, or damming up the residual toner and collecting the same into a waste toner container. As a result, for any of these cleaning methods, it has been inevitable to suffer the problems caused by the pressing of the cleaning member against the surface of the photosensitive member, for example, wear of the photosensitive member caused by the strong pressing of the cleaning member and the resultant shorter service life of the photosensitive member. From the point of view of equipment, incorporation of such a cleaning unit has naturally led to a large apparatus, and this has formed a bottleneck why trying to achieve a more compact apparatus.
From an ecological point of view, furthermore, a system not generating waste toner for effective utilization of toner, and a system having excellent fixability and offset resistance properties for energy saving have been demanded.
The disclosure of a technique conventionally known as development-simultaneous cleaning technique or cleanerless technique, as in Japanese Patent Application Laid-Open No. 5-2287, places a data point on a positive memory and a negative memory under the effect of toner remaining after transferring on an image. Now that electrophotography is widely in use, however, it is becoming necessary to transfer the toner image onto various transfer media, and in this respect, the disclosed technique does not satisfy the requirements of various transfer media.
The prior arts disclosing techniques relating to the cleanerless technique include Japanese Patent Application Laid-Open Nos. 59-133,573, 62-206,182, 63-133,179 64-20,587, 2-302,772, 5-2,289, 5-53,482 and 5-61,383. None of these techniques describes neither a desirable image forming method nor a toner composition.
Further, for development-simultaneous cleaning essentially not having a cleaning unit, a configuration for rubbing the surface of the photosensitive member with the toner and the toner carrying member is required, this tending to cause deterioration of toner, deterioration of the surface of the toner carrying member, degradation or wear of the surface of the photosensitive member caused by the use for a long period of time, and deterioration of durability. As these defects were not sufficiently solved by the conventional technology, simultaneous satisfaction of fixability and durability properties has been demanded. Now that there is a demand for a higher speed of image formation in parallel with this, the conventional technology cannot sufficiently solve problems in charging control of residual toner after transferring before collection for achieving a higher collectivity of the residual toner in development, or in keeping stability of development when reusing the collected toner in development-simultaneous cleaning technique in an apparatus having a high process speed.
Japanese Patent Application Laid-Open No. 3-259,161 provides a non-magnetic, one component developing agent by setting forth a shape factors, a specific surface area, and a particle size. The developing agent defined in this prior art does not, however, provide sufficient durability properties.
Japanese Patent Application Laid-Open No. 61-279,864 proposes a toner by setting forth values of shape factors SF-1 and SF-2. This prior art does not, however, describe copying apparatus. Moreover, a followup test of the examples revealed a low transferring efficiency and the necessity of further improvement.
Japanese Patent Application Laid-Open No. 63-235,953 processes a magnetic toner spheroidized by a mechanical impact. This prior art achieves, however, only an insufficient transferring efficiency, leaving room for further improvement.
Recently, from the point of view of environmental protection, the contact charging method of conducting charging by bringing a charging member into contact with the surface of a photosensitive member in a primary charging step and a transferring step utilizing corona discharge used conventionally, and the contact transferring method performing transferring by bringing a transferring member into contact via a transfer medium with the surface of the photosensitive member are currently receiving the main attention in the industry.
Such a contact charging method and a contact transferring method are proposed in Japanese Patent Application Laid-Open Nos. 63-149,669 and 2-123,385. These methods comprise the steps of bringing a conductive elastic roller into contact with the photosensitive member, uniformly charging the photosensitive member while applying a voltage to the conductive roller, then obtaining a toner image through subsequent exposure and developing steps, pressing another conductive roller applied with a voltage against the photosensitive member, passing a transfer medium between the photosensitive member and the conductive roller while pressing, and transferring a toner image on the photosensitive member onto the transfer medium, thereby obtaining a transferred image through a fixing step.
In the contact transferring method as described above, however, in which the transferring member is brought into contact via the transfer medium with the photosensitive member during transferring, the toner image is pressed during transferring of the toner image onto the transfer medium, thus causing a problem of a partially defective transfer known as a hollow character.
Further, as the particle size of toner becomes smaller, adherence of the toner particles to the photosensitive member (image force or van der Waals force) becomes larger as compared with the Coulomb force acting on the toner particles during transferring, thus tending to result in an increased residual toner after transferring.
The toner and the photosensitive member used in this image forming method have therefore been required to have excellent strippability properties. With a view to improving strippability of the toner from the photosensitive member and improving transferring property of the toner, use of a toner having a particle shape approximating a sphere, manufactured by the suspension polymerization method is disclosed in Japanese Patent Application Laid-Open No. 7-209,952 (corresponding to U.S. Pat. No. 5,659,857).
The toner manufactured by the suspension polymerization method has been proposed some years ago (for example, Japanese Patent Publication No. 36-10,231). The suspension polymerization method comprises the steps of preparing a monomer composition by uniformly dissolving or dispersing a polymerizable monomer and a coloring agent (and as required, a polymerization initiating agent, a binder agent, a charge control agent, and other additives), dispersing the prepared monomer composition in a continuous layer (for example, an aqueous phase), containing a dispersion stabilizing agent by means of an appropriate stirrer, and causing a polymerizing reaction, thereby obtaining toner particles having a desired particle size.
In this suspension polymerization method, in which liquid drops of the monomer composition are generated in a dispersion medium having a large polarity such as water, components having polar groups contained in the monomer composition tend to be present in the surface layer portion forming the interface with the aqueous phase, and nonpolar components are nonexistent in the surface layer portion, thereby permitting manufacture of a spherical toner having a structure known as the core-shell structure or the so called "domain-matrix" structure.
The toner based on the polymerization method now permits simultaneous achievement of low-temperature fixability, blocking resistance, and high-temperature offset resistance, which are properties contrary to each other, under the effect of inclusion of the wax component serving as a release agent.
Use of such a toner provides an advantage that the coloring agent is hardly exposed to the surface and a uniform friction charging property can be achieved. Further, because it is possible to omit the classification step, there are available such remarkable cost curtailing effects such as energy saving, reduction of time, and improvement of process yield.
However, since the toner obtained by this method has substantially a true spherical shape, defective cleaning caused by passing-through of the toner particles occurs during cleaning, particularly during blade cleaning, and this may seriously damage the quality of the transferred image.
Especially in nonmagnetic one component, the amount of charge of the toner developed on the photosensitive member leads to a large adherence (image force) of the toner particles to the photosensitive member, resulting in an increasing tendency of residual toner after transferring. The amount of charge of the residual toner after transferring tends to become larger, causing the problem of easy occurrence of defective cleaning in cleaning because of an increased adherence of the toner particles to the photosensitive member.
Also in the development-simultaneous cleaning method of conducting the cleaning step for removing residual toner after transferring simultaneously with the developing step, when using a spherical toner, use for a long period of time causes the covering additive to easily be buried in the toner surface, leading to a decreased charging property of the toner, and hence to deterioration of the image quality in many cases. This phenomenon is particularly remarkable as the process speed of the developing roller becomes higher.
It is widely known to add carbon black as a black coloring agent into the toner. Dispersibility of carbon black is largely affected by physical properties such as primary particle size, specific surface area, structure and surface quality (functional group on the surface and the like). When adding carbon black to the toner, therefore, the selection thereof exerts an important effect on the toner properties.
For example, carbon black has a smaller primary particle size and a large specific surface area as compared with the other pigments, and is hardly dispersed because of a unique structure. Insufficient dispersibility of carbon black in toner particles impairs charging property of the toner, leading to an adverse effect of causing fogging during development. In addition, the decrease in coloring ability of toner particles results in another adverse effect of a lower image concentration or greater toner consumption.
Manufacture of a toner by the use of carbon black as a pigment in the application of the foregoing polymerization method causes further problems as follows.
First, because carbon black has a functional group impairing polymerizability of a monomer such as a quinone group on the surface, the manufacture of a toner by the polymerization method leads to a lower polymerization rate with a degree of polymerization not increasing sufficiently, results in instable particles during granulation, aggregations and bonding, thus making it difficult to take out particles.
Secondly, when preparing a monomer composition by uniformly dissolving or dispersing a polymerizable and carbon black (and further, as required, a polymerization initiating agent, a crosslinking agent, a charge control agent, and other additions), carbon black is hardly dispersed in the monomer composition. When manufacturing toner particles by suspension-dispersing the monomer composition in an aqueous medium, carbon black is maldistributed, or toner particles not containing carbon black are easily produced.
Thirdly, since carbon black is conductive, a charge on the toner surface easily leaks, thus exerting an important effect on charging property of the toner. Dispersion of carbon black content, dispersibility, and aggregation between the each toner particles causes dispersion of the charging property of the toner particles, and leads to a lower developing efficiency and more serious fogging.
Such variations in carbon black content, dispersion, and aggregation between the toner particles and localization within a particle result in generation of toner particles having a high charging amount far larger than the desired one and toner particles of a low charging amount, thus causing instable charging property of the toner. Particularly in nonmagnetic one component contact developing, the presence of more toner particles of a low charging amount leads to toner splashing without adhering to, or being retained by, electrostatically the toner carrying member and contaminates the interior of the apparatus and the image: easy occurrence of toner splashing or fogging of the image. Toner particles having an unnecessarily high charging amount results in a larger adherence (image force) to the photosensitive member, which tends to cause an increase of residual toner after transferring a higher charging amount of residual toner after transferring, thus leading to easier occurrence of defective cleaning during the cleaning step. This further cause a decrease in a charging amount for the toner as a whole through prevention of charging of the other toner particles, and tends to result in toner splashing and fogging.
It is also known to add an azo-based iron complex as a charge control agent to the toner.
Japanese Patent Laid-Open No. 7-281,485 proposes use of a suspension-polymerized toner having a charging property improved by adding a specific azo-based iron complex to the toner in the non-magnetic one component developing method. This publication does not, however, describe consideration on carbon black, which has an important effect on polymerizability and charging property of the toner.
Further, in the image forming method based on development-simultaneous cleaning, no conventional art has ever solved sufficiently these problems encountered when using a toner containing carbon black.